What is an Air Barrier – Vapor Barrier – Water Resistive Barrier?

An air barrier material resists air leakage and is designed to form a continuous plane around a building to prevent uncontrolled air movement in and out of the building envelope.

What’s a vapor barrier?

Vapor barriers limit the amount of water vapor diffusing through the wall as a result of different vapor pressures. A vapor barrier does not have to be continuous, does not have to be sealed, does not have to be free of holes and does not have to be lapped.

What’s a water resistive barrier?

The function of a water resistive barrier is to keep liquid water from entering the building enclosure. Combined with flashing and other materials, the water resistive barrier ensures that there is a shingled assembly to direct liquid water to the exterior.

Confusion between Vapor barrier and Air barrier

The function of a vapor barrier is to retard the migration of water vapor, which is not typically intended to retard the migration of air. This is the function of air barriers. A vapor barrier is designed to restrict the flow of water vapor through a material, just the same as an air barrier material restricts the flow of air through a material. Remember that air leakage – and not vapor diffusion – is the real concern because air leakage accounts for over 200 times the amount of moisture transmitted by diffusion.

A single material can function as all three, an air, vapor and water resistive barrier. There are also materials that function only as a water resistive barrier that are not air barriers. Understanding the functions of the materials used is critical, so that the building performs in accordance to the Project requirements.